BEEFY: Define basic fisherman (#4328)

Related to https://github.com/paritytech/polkadot-sdk/pull/1903

For #1903 we will need to add a Fisherman struct. This PR:
- defines a basic version of `Fisherman` and moves into it the logic
that we have now for reporting double voting equivocations
- splits the logic for generating the key ownership proofs into a more
generic separate method
- renames `EquivocationProof` to `DoubleVotingProof` since later we will
introduce a new type of equivocation

The PR doesn't contain any functional changes

substrate/frame/beefy/src/equivocation.rs

@@ -38,7 +38,7 @@ use codec::{self as codec, Decode, Encode};
 use frame_support::traits::{Get, KeyOwnerProofSystem};
 use frame_system::pallet_prelude::BlockNumberFor;
 use log::{error, info};
-use sp_consensus_beefy::{EquivocationProof, ValidatorSetId, KEY_TYPE as BEEFY_KEY_TYPE};
+use sp_consensus_beefy::{DoubleVotingProof, ValidatorSetId, KEY_TYPE as BEEFY_KEY_TYPE};
 use sp_runtime::{
 	transaction_validity::{
 		InvalidTransaction, TransactionPriority, TransactionSource, TransactionValidity,
@@ -123,7 +123,7 @@ pub struct EquivocationReportSystem<T, R, P, L>(sp_std::marker::PhantomData<(T,
 
 /// Equivocation evidence convenience alias.
 pub type EquivocationEvidenceFor<T> = (
-	EquivocationProof<
+	DoubleVotingProof<
 		BlockNumberFor<T>,
 		<T as Config>::BeefyId,
 		<<T as Config>::BeefyId as RuntimeAppPublic>::Signature,

substrate/frame/beefy/src/lib.rs

@@ -41,7 +41,7 @@ use sp_staking::{offence::OffenceReportSystem, SessionIndex};
 use sp_std::prelude::*;
 
 use sp_consensus_beefy::{
-	AuthorityIndex, BeefyAuthorityId, ConsensusLog, EquivocationProof, OnNewValidatorSet,
+	AuthorityIndex, BeefyAuthorityId, ConsensusLog, DoubleVotingProof, OnNewValidatorSet,
 	ValidatorSet, BEEFY_ENGINE_ID, GENESIS_AUTHORITY_SET_ID,
 };
 
@@ -210,7 +210,7 @@ pub mod pallet {
 		pub fn report_equivocation(
 			origin: OriginFor<T>,
 			equivocation_proof: Box<
-				EquivocationProof<
+				DoubleVotingProof<
 					BlockNumberFor<T>,
 					T::BeefyId,
 					<T::BeefyId as RuntimeAppPublic>::Signature,
@@ -245,7 +245,7 @@ pub mod pallet {
 		pub fn report_equivocation_unsigned(
 			origin: OriginFor<T>,
 			equivocation_proof: Box<
-				EquivocationProof<
+				DoubleVotingProof<
 					BlockNumberFor<T>,
 					T::BeefyId,
 					<T::BeefyId as RuntimeAppPublic>::Signature,
@@ -368,7 +368,7 @@ impl<T: Config> Pallet<T> {
 	/// an unsigned extrinsic with a call to `report_equivocation_unsigned` and
 	/// will push the transaction to the pool. Only useful in an offchain context.
 	pub fn submit_unsigned_equivocation_report(
-		equivocation_proof: EquivocationProof<
+		equivocation_proof: DoubleVotingProof<
 			BlockNumberFor<T>,
 			T::BeefyId,
 			<T::BeefyId as RuntimeAppPublic>::Signature,